A synchronous motor corresponding to the above generic definition is known. It is illustrated in this document by FIGS. 1, 1a, 2 and 3 which set forth the state of the prior art.
FIG. 1 is a partial section in a linear synchronous motor constructed according to the prior art. Here, the armature 60 is a stator that consists of a pack of ferromagnetic laminations and in which the yoke 1 and teeth 10 may be seen, the teeth 10 being regularly arranged with a tooth pitch .tau.n. The armature 60 further comprises a plurality of slots 2 separating the teeth 10 and in which are housed coil windings 4, the latter being insulated from the yoke 1 and from the teeth 10 by insulating sheets 3. In this type of motor, the heads of teeth 10 are terminated by a broadened portion or shoe 10' whose purpose is to create an opening or pre-slot 6 of reduced width for reasons that will become apparent below. The windings 4 are generally loosely coiled, in no precise order either with a spooling machine or inserted by hand into the slots 2 through the pre-slots 6. To keep the winding in slot 2, a sliding closer 5 is provided that bears on the shoes 10'.
FIG. 1 also shows the motor's inductor 61 which in the present example is the movable part of the motor that travels linearly along the axis y. Inductor 61 essentially comprises a plurality of permanent magnets 8 in the shape of rectangular parallelepipeds that are regularly disposed, with a pole pitch .tau.p, on a plane flux-returning sole 9 made of ferromagnetic material. The armature 60 and inductor 61 are separated by an air-gap 7.
FIG. 2 shows the distribution of the normal component of the magnetic induction B expressed in Tesla (T) and which extends here over two pole pitches .tau.p. It should be noted that induction B is that which is produced solely by the magnets 8 of inductor 61 on the teeth 10 of armature 60, whether the coils 4 are energized or not. It will be seen in FIG. 2 that the openings 6 of slots 2 cause disturbances 15 and 16 that are clearly visible in the outline of induction B. These openings, of width bn, are responsible for a phenomenon that is well known in permanent-magnet motors called the reluctant effect. This effect creates a parasitic force, or reluctant force Fr, which is directed along axis y and which disturbs the motor's proper operation. The variations of this force Fr is represented in the graph of FIG. 3.
The graph of FIG. 3 is based on a motor having a tooth pitch .tau.n of 12 mm, a pole pitch .tau.p of 16 mm and a slot opening bn of 1.5 mm, the ratio bn/.tau.n thus having a value of 0.125. The pole pitch .tau.p is shown along the abscissa and the reluctant force Fr along the ordinate. .tau.p is expressed in millimetres (mm) and Fr in Newtons (N). The curve 17 of the FIG. 3 graph reflects the outline of the reluctant force Fr that would be produced if the armature only had one slot. This curve is characterized by two unstable points 22 and 22' of low steepness and by a stable point 21 of high steepness. At points 22 and 22' slot 6 is in the middle of a magnet 8 (-.tau.p/2 and +.tau.p/2), whereas at point 21, it is between two magnets 8. If the inductor 61 is for instance located between 0 and 3.2, it will move to the right in supplying a driving force (slope 19) and stabilize itself at point 21. But if the inductor 61 is located between 16 and 12.8, it will move to the same point 21 in supplying a braking force (slope 20). The curve 18 of the FIG. 3 graph reflects the outline of the total reluctant force Fr that is produced over one pole step .tau.p, the armature 60 having eight slots. The parasitic reluctant force then has a succession of eight parasitic maxima that disturb the proper operation of the motor, these forces being of the order of 16 Newtons. It will thus be appreciated that if no due care is given to this phenomenon, the motor may become unusable, as the reluctant force can then exceed the maximum force that can be provided by the motor when supplied with current.
To eliminate or greatly reduce this reluctant effect, one widespread technique consists in staggering the laminations forming the armature 60 in relation to one another so that in a section such as that represented in FIG. 1a, the longitudinal axes of the teeth 10 and of the slots 2, shown in chain-dotted lines, form an angle other than 90.degree. with the direction of motion y of inductor 61, not shown in FIG. 1a, in relation to armature 60. It should be noted that, in FIG. 1a, which is a section along axis A--A of FIG. 1, the windings 4 and the insulating sheets 3 disposed in the slots 2 have not been shown, and that the laminations forming the armature 60 have not been illustrated separately.
This technique of staggering the laminations forming armature 60 gives rise to additional difficulties by complicating the tooling required for manufacturing purposes and by making it more difficult to insert the windings 4 into the slots 2.
Another technique, which may be combined with the previous one, consists in disposing the magnets 8 obliquely, i.e. in a manner such that the arrises thereof lying parallel to the plane of sole 9 respectively form angles other than 0.degree. and 90.degree. with the direction of motion y of inductor 61 in relation to armature 60. This technique also complicates the manufacture of the motors.
In any case, besides the above-mentioned arrangements, it will always be endeavoured to provide the pre-slots 6 with a width bn that is as small as possible, thereby complicating the spooling operations since, because of the very small slot width bn, the coils, before being fitted, must be arranged loosely to enable them to be inserted into the slot 2 through the pre-slot 6. This looseness for packing purposes means that the wires forming windings 4 are very irregularly arranged in slots 2. Consequently, the space filling coefficient of slots 2 is low (of the order of 30%) and the thermal resistance between the windings and the yoke 1 is large. These drawbacks respectively lead to low motor efficiency and to poor thermal capacity.